See this post for an explanation.
They are queries made by Pi-hole itself (pi.hole), and by the OS on which Pi-hole is installed (localhost) if that OS is using Pi-hole (ie, itself) for DNS. They will use IPv6 if IPv6 is available to use, for example if the Pi-hole's OS is running it and other clients on the network are too, especially if the router supports it and it is enabled there.
Yes, especially if conditional forwarding is enabled. Nothing to be concerned about.